In a typical radio communications network, wireless terminals, also known as mobile stations, terminals and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station or network node, e.g. a radio base station, RBS, which in some networks may also be referred to as, for example, “NodeB”, “eNB” or “eNodeB”.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations/network nodes connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially flat architecture comprising radio base station nodes without reporting to RNCs.
Orthogonal Frequency Division Multiplexing, OFDM, is a frequently used modulation scheme in modern wireless communication networks. A major reason for its popularity is the orthogonality it provides; that is, two different resource elements, REs, such as, for example, two different QAM symbols, do not interfere with each other as long as the channel delay spread is shorter than the Cyclic Prefix, CP. The channel delay spread refers to the time span of the channel impulse response, i.e. how much a transmitted delta pulse has been spread out in time when it reaches the receiver.
An example of a frame structure for such an OFDM modulation scheme is illustrated in FIG. 1. FIG. 1 shows a system subframe, i.e. the minimum scheduling unit in time employed in the wireless communication network. Here, the system subframe is exemplified as comprising 4 complete OFDM symbols. It also shows REs for each complete OFDM symbol and subcarrier. The duration of a complete OFDM symbol is denoted TOFDM, and the duration of the Cyclic Prefix, CP, is denoted TCP.
In particular, OFDM allows for using an arbitrarily small or large subset of the REs for transmitting reference signal information, i.e. reference symbols, RSs, which also may be referred to as pilots or pilot symbols. The number of RSs may hence be chosen so as to carefully balance the need for RSs, which are used to achieve a good channel estimation performance, against the desire to minimize RS overhead signalling. This is also true for several other multi-carrier modulation schemes, such as, e.g. Filter-Bank Multi-Carrier, FBMC, modulation. However, in some important uses of OFDM-based modulation scheme, it is not possible to have a fine-granular control over the amount of RSs, which may possibly result in a large RS signalling overhead.
A first example is DFT-spread OFDM modulation. Here, Quadrature Amplitude Modulation, QAM, symbols are subjected to a Discrete Fourier Transform, DFT, before being modulated using OFDM modulation. In this case, each RE carries a linear combination of all QAM symbols in the OFDM symbol, and replacing individual REs by RSs is not possible without severely disturbing all data in the OFDM symbol. Furthermore, replacing individual QAM symbols before the DFT is also not viable in this case, since the inter-symbol interference is then large. Although this inter-symbol interference may be significantly reduced by equalization, but in order to perform the equalization, accurate channel estimation based on the RSs first has to be performed. This in turn depends on non-interfered RSs. Hence, instead an entire OFDM symbol will have to carry only RSs.
As illustrated in FIG. 1, it follows that if the system subframe, i.e. the minimum scheduling unit in time employed in the wireless communication network, is only a few OFDM symbols, e.g. four as in FIG. 1, and each system subframe must contain RSs, i.e. which is the normal typical case in order to enable reliable demodulation, then the RS signalling overhead will be large, i.e. in this example the RS signalling will be about ¼ or 25%.
A second example is when performing spatial beamforming, BF, with a multi-antenna transmitter under certain common hardware constraints. More precisely, if there is only one digital transmit chain, e.g. for cost and power consumption reasons, and the beamforming is performed using only analog full-bandwidth phase shifters at the individual antenna elements, then each OFDM symbol may only transmit a signal in one spatial direction. This is an issue since it means that the search for the best beam direction to use for data transmission, i.e. a scan over all beam directions which may comprise e.g. possibly hundreds of potential beam directions, only may be performed at a rate of one direction per OFDM symbol. This means that no data transmission may take place on those OFDM symbols. Hence, the result is a very large RS signalling overhead.